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Tuesday, 9 June 2015

CONTROL LAGS

The
primary concern in most process control systems is the degree of accuracy in
responding to process changes and the time it takes to achieve the desired
correction.

Control
accuracy is dependent upon the proper selection, calibration, and operation of
the instruments and control systems.Factors
affecting the degree of accuracy (process response time) are:

SProcess Lag.

SMeasurement Lag.

STransmission Lag.

SResponse Lag.

Lag
is a process term for delay.In other
words, delays in measurement, transmission of control signals, and the process
itself to respond to changes.All effect
the process response time.The degree of
accuracy of any process control system can be maximised by reducing these lags.

When
the response time for the system is not suitable for the process requirements,
design changes are usually made.These
design changes can include the addition of booster relays, or modifying the
control loop to maximise the degree of system accuracy.

Process Lag

Process
Lag is the time difference between the control elements set point being changed
and the amount of time it takes for the process variable (PV) to change in
response to the change.

As
shown in Figure 2-1, a heat exchanger is operating at a temperature of 200°C.If the operator wishes to increase its
temperature to 300°C, he must allow more steam (or Heat Transfer Fluid) to flow
through the exchanger.However, the
temperature in the heat exchanger will not rise to 300°C immediately after he
increases the steam flow.

The
steam flow changes from one steady value to another quite quickly but the
change in the process fluid temperature is gradual.The process characteristics that cause the
process fluid temperature to change gradually instead of instantly are the
resistance, capacity and dead time of the process fluid (see below).This time difference is the process lag.

Figure
2-2 shows how the response curve for the process fluid temperature
"lags" behind the response curve for the steam flow.

Process
Resistance

Process
resistance can be defined as the opposition to the transfer of energy or flow
through the process.

Capacity
of Process

Process
capacity is the ability of a process to hold energy (or a quantity of
material).

Dead
Time

Dead
time is the interval from the time that a control element change takes place
until its effect is felt on the control variable as shown in Figure 2-2.

The
higher the capacity of a process, process resistance and dead time, the higher
will be the process lag and vice versa.

Measurement Lag

Measurement
lag is the time it takes the measuring device to give a signal which accurately
represents the process variable.

In
most cases, measurement of flow, level, and pressure are not greatly affected
by measurement lag.This is because most
of the devices used to measure these variables respond quickly to process
changes.

The
measurement of temperature, however, is often subjected to time delay, or
measurement lag.This measurement lag is
usually due to the way in which heat is transferred to the process material and
through the process equipment.It is
also due to the way in which heat is transferred through the sensing element.

The
net effect of these factors is that there will usually be a time difference
from the moment the process temperature changes, to the moment the sensing
element transmits this information.

In
this case, let us consider that anything slowing the flow of heat from the
process to the fluid in the filled bulb is a resistance.Each resistance between the process material
and the fluid in the bulb will have to be overcome before the element can sense
any temperature change.

As
you can see in this example, the process temperature must overcome four
resistances before the temperature can be sensed by the filled thermal element.

·First, the temperature
change must pass through the walls of the thermowell.

·Then it must pass through
the space between the thermowell and the filled bulb.

·Then it must pass
through the walls of the bulb.

·Finally, it must change
the temperature of the fluid in the bulb.

Once
the temperature begins to change in the fluid, the filled thermal element will
transmit the process change to a temperature transmitter.

Transmission
Lag

Transmission
Lagis the time interval between a
signal being transmitted from the detecting element to the controller then to
the final control element.

Transmission
lag is more evident in pneumatic control systems than in electronic control
systems.This is because pneumatic
signals travel through the control loops at a slower rate than electronic
signals.Factors affecting transmission
lag in pneumatic control loops are:

a)distance between the
instruments in the loop

b)size of the pneumatic
tubing

c)signal pressure

You
can see from Figure 2-4, that the distance between a transmitter in the field
and the control room will affect the time it takes for a pneumatic signal to
reach the controller.The same factor
applies when the controller sends its output signal to a control valve in the
field.The greater the distance, the
longer it will take for the controller to receive the signal.

To
correct this problem, pneumatic control loops can be designed to minimise
transmission lag.This is usually
accomplished by increasing the diameter of the pneumatic tubing and increasing
the transmission signal pressure for long distance tubing runs.Booster relays can also be used to compensate
for pressure losses on long tubing runs.In this way transmission lag can be kept to a minimum in pneumatic
control loops.

Response Lag

This
is the time interval between the arrival of the signal to the control valve and
the time the control valve responds to the signal (moving towards opening or
closing).

This
lag is affected by many factors.Some of
these factors are:

a)The strength of the
spring in the valve actuator.

b)The friction between
the valve stem and the packing.

c)The fluid pressure on
the valve plug.

Response
lag can be minimised by increasing the air pressure to the actuator.This is achieved by using a valve positioner
on the actuator.

Having
studied the different lags that can occur in a control system, let us now see
how they affect the system as a whole during a process load change.

These
changes will usually be due to changes in the process condition, or changes in
the operation of the process.How quickly
the control system responds to these load charges will depend upon the various
control lags in the system.

Figure
2-5 shows a typical example of how a control loop responds to process load
changes.

Let
us see how the process temperature increases:-

Because
it takes time for the temperature to increase throughout the entire volume of
the heat exchanger, there will be some dead time before the temperature
increase reaches the sensing element.Then,
there will be the process lag for the temperature to reach a new value.

Once
the temperature change reaches the sensing element, there will be a measurement
lag as it overcomes the various resistances of the thermowell and the filled
thermal element.When the change is
finally sensed, it is then sent to the transmitter where it is converted to a 4
to 20 mA electronic (0.2 to 1.0 bar pneumatic) signal.This signal is then transmitted to the
temperature controller.

The
controller output signal is then transmitted to the control valve.Both the transmitter signal and the
controller output signal takes time to pass through the transmission lines.This is the transmission lag.When the signal reaches the control valve,
the control valve will take some time to respond to the new signal by opening
or closing.This is the response lag.

Due
to all these delays, the correction of the process fluid outlet temperature to
the set-point will take time.For this
reason the process temperature (the controlled process variable) fluctuates up
and down for some time before it returns to the set point.